The present invention is in the field of light-emitting polymers and light emitting devices produced therefrom.
Conjugated polymer based light-emitting devices have become an extensive area of academic and industrial research since the report of electroluminescence (EL) in poly(phenylene vinylene) (PPV) in 1990 [1].
A great number of different conjugated polymers have been found to exhibit EL including PPVs [1-3], poly(phenylphenylene vinylene) [4], polyphenylenes [5-7], polythiophenes [8-9], polyquinolines [10], polypyridines [11-12], poly(pyridyl vinylenes) [12-14] and many copolymers of these materials.
In addition to many different materials, numerous configurations have been used to change and improve device performance. For instance, the use of additional layers to improve device efficiency has been known for some time [2,15]. Inserting a hole-transport (electron blocking) layer between the anode and emitting polymer or an electron-transport (hole-blocking) layer between the cathode and emitting polymer can greatly improve efficiency by confining the majority carrier to the emitting layer. A well known hole-transport (electron blocking) layer is poly(vinyl carbazole) (PVK) which has a large band gap (3.5 eV) and is itself luminescent [16-18].
Despite these advances there remains a need for improvements in the electroluminescence performance of light emitting polymers. Particularly, there remains a need to improve the performance of exiplex-forming bilayer devices so as to reduce or eliminate the redshifiing believed to be associated with the aggregation of polymeric chains within the emitting polymer.
It is a goal of the present invention to produce light emitting polymer and light emitting polymer devices made which give light emissions having reduced redshifting.
In view of the present disclosure or through practice of the present invention, other advantages may become apparent.
In general terms, the present invention includes a light emitting polymeric material the light emitting polymeric material capable of producing electroluminescence upon being provided with a flow of electrons, the light emitting polymeric material comprising a plurality of polymeric chains comprising polymeric chains each having substituent moieties of sufficient number and size and extending from the polymeric chain and about a substantial portion of the circumference about the polymer chain so as to maintain the polymeric chains in a sufficiently deaggregated state (referred to herein as a xe2x80x9cstrappedxe2x80x9d polymer), so as to substantially prevent the redshifting of the electroluminescence and the lowering of light emission efficiency of the electroluminescence.
The electron transporting polymer may include polymeric chains selected from copolymers having the structure: 
Some specific examples of polymers used in the present invention include: 
The light emitting polymeric material may also be used in single layer, bilayer or other multiple layer devices, using the polymeric material of the present invention. In the case of a single polymeric layer device, the polymeric material of the present invention may be used as the electron transporting/electron blocking layer. In the case of a bilayer or multi-layer devices, the polymeric material of the present invention may be used as the electron transporting layer in conjunction with an electron blocking layer of another appropriate polymer, such as might be selected from the group consisting of poly(vinylcarbazole).
The present invention also includes a light emitting device comprising a light emitting polymeric material according to the present invention in all of its embodiments, and a source of electrical current so as to supply the light emitting device with a flow of electrons capable of producing electroluminescence from the device.
Other examples of synthetic methods are described in reference 22 below.
These devices may be constructed in accordance with deposition and assembly techniques known in the art. The present invention may be used in the creation of a wide variety of lighting and lighted displays, giving the many advantages associated with polymeric materials.
In accordance with the present invention, results are presented for bilayer devices using PVK as a hole-transport layer and a family of copolymers of PPV and poly(pyridyl vinylene) PPyV with various side groups as the emitting layers. The absorption, photoluminescence and electroluminescence spectra indicate that the PL and EL are attributed to the formation of an exciplex at the PVK/copolymer interface for all the copolymer systems studied. An exciplex, like an cxcimer, is an excited state complex, except that an exciplex is formed between two different molecules (polymers in this case) rather than identical ones for an excimer [19]. Contrary to expectations, earlier reported devices do not exhibit exciplex formation. For example, Greenham el al reported a bilayer device with CN-PPV and PPV, but the EL matches the PL and EL of a single CN-PPV film [3]. Results for other bilayer configurations also do not support exciplex formation [2]. Osaheni and Jenekhe [20] have observed exciplex formation in bilayers of PBO and TTA, but only for PL, although they do suggest that exciplexes may be important processes in organic light-emitting devices [20-21]. PL and EL due to exciplex formation has been reported in blends of PVK and a multiblock copolymer by Karasz and coworkers [17]), but devices with separate layers were not investigated.